1. Technical Field
The present invention relates to a method of manufacturing an electrode of a molten carbonate type fuel cell.
2. Background Art
A lot of fuel cell systems have been developed and a molten carbonate fuel cell is one of typical fuel cells. A conventional molten carbonate type fuel cell includes an electrolyte plate (tile) soaked with molten carbonate, a cathode (oxygen electrode) and an anode (fuel electrode). The cathode electrode and the anode electrode sandwich the electrolyte plate. Oxidizing gas is fed to the cathode electrode and fuel gas is fed to the anode electrode to produce a potential difference between the two electrodes. Generally, a plurality of sets of electrolyte plate, cathode electrode and anode electrode are stacked one after another via separators.
One of the manufacturing methods of green tapes of electrode is a tape casting method using a doctor blade. The tape casting method is a continuous producing method so that it is suitable for a manufacturing of large electrodes and a mass production of the green tapes of electrode. The tape casting method is also preferable to form electrodes of homogeneous thickness.
With the tape casting method using the doctor blade, the electrode is manufactured as follows: Raw material powders, a dispersant and an organic solvent are mixed by a ball mill until the raw material powders are dispersed to primary particles. Then, a binder and a plasticizer are added to the primary particles and mixed with each other until they become slurry. The slurry is casted to a tape-like (sheet-like) substance by the doctor blade. The tape-like substance is then dried to a green tape and the green tape is sintered to a porous electrode.
There are four types of raw material powder used in the above-described method. Those are: (i) Ni powders only; (ii) Ni-Cr or Ni-Al alloy powders; (iii) Ni powders and Cr powders (Cr content is 1 to 10%); and (iv) Ni and oxides (for example, Al.sub.2 O.sub.3). These powders, however, have following disadvantage respectively:
(i) Ni powders
When an electrode is made from Ni powders only, the electrode has a low anti-creep strength since the electrode consists of a single substance. The electrode, therefore, is easy to deform during the operation of the fuel cell;
(ii) Ni-Cr or Ni-Al alloy powders
It is difficult to manufacture fine powders of the alloy using the atomizing method, though the electrode needs to consist of fine particles or grains in order to obtain a desired microstructure. The sintering process, moreover, needs to include an oxidizing step and a reducing step, to obtain a suitable microstructure, as long as the atomizing method is employed. With this method, the cost of the manufacturing of the raw alloy powders and the cost of sintering of the electrode are high;
(iii) Ni powders and Cr powders (Cr content is 1 to 10%)
The Ni powders and the Cr powders are segregated, even though the Ni and Cr powders are mixed and sintered; and
(iv) Ni and oxides (e.g. Al.sub.2 O.sub.3)
After the tape casting, the oxides tend to be segregated on an upper layer of the tape because of the large difference of density between the Ni powders and the oxide powders.
It should be noted that the powders are likely to be segregated because of the difference of the densities of the powders (The density of Ni is 8.89 g/cm.sup.3 ; Cr is 7.19 g/cm.sup.3 ; oxides is 2-4 g/cm.sup.3 ; and Al-intermetallic compound is 3-6 g/cm.sup.3).
Meanwhile, it is generally said that a microstructure of the electrode needs to be stable in order to improve the performance of the fuel cell. The microstructure, however, changes with the creep deformation of the electrode. Therefore, Cr is added, as the powders of (ii) and (iii), to improve a creep strength of the electrode. However, even though Cr is added 8 to 10%, the electrode is deformed by the creep and the microstructure of the electrode is changed. More specifically, in case of (iii), the Cr powders mixed with the Ni powders dissolve into Ni powders during the sintering, and the Cr changes into Cr oxide on the Ni-Cr alloy under the MCFC condition, which reinforces the anode electrode. Generally, it is necessary to add 8-10 wt. % of Cr in the electrode to improve the anti-creep strength of the electrode, and more Cr would be added to reinforce the electrode more. However, it is not recommended to add a large amount of Cr since the Ni-Cr alloy is lithiated and consumes the carbonate during the power generation. In addition, in case of (iv), the Al.sub.2 O.sub.3 powders dispersed into the Ni powders reinforces the anode electrode as mentioned before. However, it is not easy to sinter Al.sub.2 O.sub.3 and Ni together. Even if pure Al powders are added to the Ni powders, the Al powders are easy to be oxidized into Al.sub.2 O.sub.3 during the sintering and then Al does not dissolve into Ni.